Method and system for producing steel having low nitrogen content

ABSTRACT

A method of introducing a denitrogenizing flux material into a container of molten metal includes steps of providing a wire-like vector that has an inner core of flux material and an outer layer that is used to contain the inner core, and using a wire feed machine to introduce the wire-like vector into a container of molten metal. In comparison with conventional top slag removal techniques, this process permits the material from which the inner core is made to be injected into the container of molten metal at a speed and direction that promotes a controlled mixing of the flux material and the molten metal.

[0001] This is a continuation-in-part of Ser. No. 08/866,173, filed May 30, 1997 and a continuation-in-part of Ser. No. 08/979,771, filed Nov. 26, 1997, the disclosures of which are hereby incorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates in general to the field of metallurgical processing, and more specifically to processes for adding materials, such as fluxes, into molten metal such as liquid steel.

[0004] 2. Description of the Prior Art

[0005] In the production of metals such as steel, it is sometimes desirable to remove unwanted trace elements from the liquid metal by reacting one or more flux materials with the liquid metal.

[0006] For example, nitrogen is generally considered to be an unwanted element in steel. Nitrogen enters into liquid steel from the air and from contaminants, such as oil, that may find their way into the raw and recycled material from which steel is made. The nitrogen changes the mechanical properties of steel, making it harder and less ductile. It can also chemically combine with aluminum, or other elements, to form inclusions, affecting the quality of the product. Combined compounds can also migrate to grain boundaries in the steel's microstructure, weakening the steel at elevated temperatures, giving rise to inter-granular cracks.

[0007] Nitrogen levels are particularly a problem in steel that is produced by the so-called “mini-mills,” which generally use electric arc furnaces to melt the steel and that also tend to use a relatively high level of metal scrap as source material. It is not uncommon to see steels that are produced at such facilities as having a nitrogen content that is within the range of about 60 parts per million (ppm) to about 120 ppm. Steels that are made in mills having a basic oxygen furnace, on the other hand, have a nitrogen content that is commonly within the range of about 30 ppm to about 50 ppm. Some specialty applications, such as for the automotive body, however, require nitrogen levels that are as low as 20 ppm. Some facilities use vacuum degassing equipment, which essentially exposes the liquid steel to near vacuum conditions to decarburize the steel. Some degree of nitrogen removal may be achieved as a by-product of this process. Unfortunately, this process is expensive and is not able to extract nitrogen that has already chemically combined with other elements, such as aluminum.

[0008] More recently, it has been proposed to use fluxes to remove nitrogen from molten steel by adding a synthetic ladle slag of appropriate composition to the top surface of the molten steel within a ladle. The top slag process, which is also in common practice for desulfurizing steel, involves heating the steel within the ladle for an extended period of time and to circulate the steel, thereby exposing all the molten metal over time to the liquid metal-slag reaction interface. While denitrogenization with top ladle slag is promising in the sense that it permits reduction of nitrogen to levels otherwise not achievable by other processes, it has several practical limitations and consequently it is not in wide practice at this point. The top ladle slag denitrogenizing treatment would require skimming of carried over furnace slag from the ladle and introduction of a synthetic denitrogenizing ladle flux of a specific composition on top of liquid steel. Adding of such flux to the ladle already containing other slag would not be desired and effective due to the dilution effect by the other slag. Effects of nitrogen removal by doing so would be questionable due to variability of composition of diluted ladle slag. The skimming operations, which are not uncommon in some practices such as special desulfurization processes, are very time consuming and not energy efficient. Temperature loss of steel in the ladle not covered with slag can amount to 100-150 degrees F depending on the type of operation. Addition of solid slag fluxing mix requires extended heating to melt and bring the mix into solution. This requires a great deal of time and energy, both of which are expensive factors in the overall cost of production.

[0009] Denitrogenization using fluxes is being explored in several universities on experimental scale. The removal of nitrogen from steel appears to take place both in acidic and basic fluxes. The nitride capacity of fluxes has a V-shaped dependency on the optical basicity. The nitride capacity is high at low optical basicity; as the optical basicity is increased it reaches a minimum and starts to increase later. This behavior is explained by Sommerville et. al. to be related to the structural effects; the nitrogen which substitutes for oxygens in the network shows an inverse relationship with basicity whereas that replacing “free” oxygens is directly related to basicity. While the knowledge of denitrogenization with fluxes is improving, the techniques used in these studies are on a laboratory scale and have employed the top slag method. As discussed above this technique has several practical limitations for routine technical uses.

[0010] Articles addressing flux denitrogenization, the details of which are incorporated into this document as if set forth fully herein, are as follows: Studies on Slags for Nitrogen Removal from Steel, J. P. Ferreira et al., [75th Steelmaking Conference, Iron &Steel Society, Apr. 5-8, 1992, Toronto, Ontario, Canada—Abstracts], pp. 216-217; Studies of Nitrogen in Steel in a Plasma Induction Reactor with a BaO—TiO ₂ Slag, L. B. McFeaters et al., [75th Steelmaking Conference, Iron &Steel Society, Apr. 5-8, 1992, Toronto, Ontario, Canada—Abstracts], pp. 218-219; and The Behavior of Nitrogen During Plasma-Enhanced Refining, M. Takahashi et al., [75th Steelmaking Conference, Iron Steel Society, Apr. 5-8, 1992, Toronto, Ontario, Canada—Abstracts], pp. 220-221; and Synthetic Slags for Nitrogen Removal, J. P. Ferreira, I. D. Sommerville, and a. Mclean, [Iron and Steelmaker, May 1992], pp. 43-49; and The use and Misuse of Capacities in Slags, I. D. Sommerville, A. Mclean and Y. D. Young [Proceedings International Conference on Molten Slags, Fluxes and Salts, 1997 Conference], pp. 375-383; and Solubility of Nitrogen in Cao—Sio ₂ —CaF ₂ Slag Systems, H. S. Song, D. S. Kim, D. J. Min and P. C. Rhee [Proceedings International Conference on Molten Slags, Fluxes and Salts, 1997 Conference], pp. 583-587; and Nitride Capacities in Slags, H. Suito, K. Tomioka, and J. Tanabe, [Proceedings of 4th International Conference on Molten Slags and Fluxes, 1992, Sendai], pp. 161-166.

[0011] A need exists for an improved system and process for introducing a denitrogenizing flux to a quantity of molten metal, such as steel, in a manner that is less time consuming and less wasteful of energy than methods of flux addition and mixing that are in conventional use.

SUMMARY OF THE INVENTION

[0012] Accordingly, it is an object of the invention to provide an improved system and process for introducing a denitrogenizing flux to a quantity of molten metal, such as steel, in a manner that is less time consuming and less wasteful of energy than methods of flux addition and mixing that are in conventional use. In order to achieve the above and other objects of the invention, a method of introducing a denitrogenizing flux to an amount of molten metal, includes, according to a first aspect of the invention steps of: (a) encasing the denitrogenizing flux with an outer layer of a metallic material of equal of lower melting point in comparison to the liquid metal; and (b) introducing the flux so encased into the molten metal, whereby the outer layer will melt, thereby introducing the flux into the molten metal.

[0013] According to a second aspect of the invention, an article for introducing a denitrogenizing flux to an amount of molten metal includes an outer layer of a metallic material that has a melting point that is beneath the anticipated temperature of the amount of molten metal; and a denitrogenizing flux that is encased within the molten metal, whereby the outer layer will melt after the article has been introduced into the molten metal for a predetermined period of time, thereby permitting introduction of the denitrogenizing flux into the molten metal at a depth below the top surface of the molten metal. According to a third aspect of the invention, a method of denitrogenizing an amount of molten metal includes steps of: (a) providing an amount of molten metal; and (b) introducing a denitrogenizing flux into the molten metal in such a way that the flux becomes exposed to the molten metal at a location that is at a depth that is substantially below the top surface of the molten metal, thereby promoting more efficient mixing of the flux into the molten metal.

[0014] A method of introducing a denitrogenizing flux to an amount of molten metal, includes, according to a fourth aspect of the invention, steps of: (a) supplying an amount of denitrogenizing flux into a lance assembly of the type that includes a nozzle that is constructed and arranged to be immersed in molten metal; and (b) using the lance assembly to introduce the flux into the molten metal.

[0015] These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic depiction of a conventional wire feed machine, which is shown in operation according to the invention;

[0017]FIG. 2 is a cross-sectional view taken along lines 2-2 in FIG. 1;

[0018]FIG. 3 is a schematic depiction of a system constructed according to an alternative embodiment of the invention; and

[0019]FIG. 4 is a schematic control diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0020] Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIG. 1, an improved system 10 for producing steel that has a low nitrogen content includes a source 12 of a wire vector 14 that is constructed and arranged to introduce a denitrogenizing flux into molten metal such as steel. System 10 utilizes a conventional wire feed machine of the type that includes feeding structure 16 for feeding the wire vector into a guide chute 18 at a controlled velocity so as to cause the wire vector 14 to penetrate into the molten steel 22 at a predetermined speed and direction.

[0021] As may be seen in FIG. 2, the wire vector 14 includes an outer layer 24 of a material, such as steel, that has a melting point that is at or beneath the temperature of the molten metal 22. Preferably, the outer layer 24 is fabricated from steel a material with equal or lower melting point than the liquid melt, preferably the outer layer can be made of steel or aluminum. Outer layer 24 thus encases the nonmetallic substance in an elongated, tube-like hollow cladding of metallic material that is designed to melt after being introduced into the molten metal 22.

[0022] Wire vector 14 further includes an inner body of a powdered denitrogenizing flux material 26, which includes calcium oxide (CaO) and at least one compound selected from the group consisting of oxides, silicates, carbonates of alkali and alkaline earth metals and oxides, fluorides, silicates and carbonates of metals selected from the group consisting of Calcium (Ca), Silicon (Si), Magnesium (Mg), Boron (B), Titanium (Ti), Barium (Ba) and Aluminum (Al). The most preferred flux materials are CaO—BaO—TiO₂—(Al₂O₃), CaO—TiO₂(Al₂O₃) and Calcium-Boron oxide bearing fluxes. Alternatively, any other flux that is capable of achieving the desired denitrogenization could be substituted.

[0023] A process according to one embodiment of the invention involves encasing the denitrogenizing flux 26 with the outer layer of metallic material 24 and introducing the flux 26 so encased into the molten metal 22, whereby the outer layer will melt, thereby introducing the flux into the molten metal.

[0024] Another embodiment of the invention is depicted in FIGS. 3 and 4. Referring in particular to FIG. 3, a system 30 for introducing a denitrogenizing flux 20 to an amount of molten metal 32 that is constructed according to a preferred embodiment of the invention includes a container 34, such as a ladle, for holding an amount of molten metal 32 such as liquefied steel. System 30 further includes a lance assembly 36 that is preferably inclusive of a container or hopper 38 of a supply of denitrogenizing flux 40, and a lance 42 for introducing the flux 40 into the molten metal 32.

[0025] Preferably, the flux material 40 is a powdered denitrogenizing flux material which includes calcium oxide (CaO) and at least one compound selected from the group consisting of oxides, silicates, carbonates of alkali and alkaline earth metals and oxides, fluorides, silicates and carbonates of metals selected from the group consisting of Calcium (Ca), Silicon (Si), Magnesium (Mg), Boron (B), Titanium (Ti), Barium (Ba) and Aluminum (Al). The most preferred flux materials are CaO—BaO—TiO₂—(Al₂O₃), CaO—TiO₂—(Al₂O₃) and Calcium-Boron oxide bearing fluxes. Alternatively, any other flux that is capable of achieving the desired denitrogenization could be substituted.

[0026] A pressure source 44 of an inert gas, preferably argon, is communicated with a first end of the lance 42, and a control valve 46 is interposed between the pressure source 44 and the lance 42 in order to control the flow of the inert gas through the lance 42. A second end of the lance 42 terminates in a nozzle 48, which during operation of the system 30 is immersed in the molten metal 32. The portion of the lance 42 that is expected to be immersed in the molten metal 32 during operation is encased in a protective refractory sleeve 54, as is shown in FIG. 3.

[0027] A conveyor 50 that is powered by a motor 52 is positioned to supply flux material from the hopper 38 into the lance 42 at a location that is between the valve 46 and the nozzle 48. As may be seen in FIG. 4, System 30 includes a control system having a CPU 56 that controls operation of the motor 52 and the valve 56.

[0028] In operation, system 30 is operated to introduce the denitrogenizing flux 40 into the molten metal 32 by CPU 56 instructing motor 52 to cause conveyor 50 to move flux into the lance 42, and by opening valve 46, thus causing the flux 40 to become entrained in the flow of inert gas that is provided by the pressure source 44. The flux is then injected into the molten metal 32 at a preselected depth and velocity that is chosen to promote fast, efficient mixing of the flux 40 with the molten metal 32. Accordingly, the invention adds denitrogenizing flux in a manner that is less time consuming and less wasteful of energy than methods of flux addition and mixing that are in conventional use.

[0029] It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A method of introducing a denitrogenizing flux to an amount of molten metal, comprising steps of: (a) encasing the denitrogenizing flux with an outer layer of a metallic material that has a melting point that is beneath the temperature of the amount of molten metal; and (b) introducing the flux so encased into the molten metal, whereby the outer layer will melt, thereby introducing the flux into the molten metal.
 2. A method according to claim 1 , wherein step (a) is performed by encasing the flux in an elongated, tube-like hollow cladding of metallic material, thereby forming a wire-like vector.
 3. A method according to claim 2 , wherein step (b) is performed by introducing the wire-like vector into the molten metal by using a conventional wire feeding machine.
 4. A method according to claim 1 , wherein said denitrogenizing flux includes calcium oxide (CaO) and at least one compound selected from the group consisting of oxides, silicates, carbonates of alkali and alkaline earth metals and oxides, fluorides, silicates and carbonates of metals selected from the group consisting of Calcium (Ca), Silicon (Si), Magnesium (Mg), Boron (B), Titanium (Ti), Barium (Ba) and Aluminum (Al).
 5. An article for introducing a denitrogenizing flux to an amount of molten metal, comprising: an outer layer of a metallic material that has a melting point that is beneath the anticipated temperature of the amount of molten metal; and a denitrogenizing flux that is encased within the molten metal, whereby the outer layer will melt after the article has been introduced into the molten metal for a predetermined period of time, thereby permitting introduction of the denitrogenizing flux into the molten metal at a depth below the top surface of the molten metal.
 6. An article according to claim 5 , wherein said outer layer encases the nonmetallic substance in an elongated, tube-like hollow cladding of metallic material, thereby forming a wire-like vector.
 7. An article according to claim 5 , wherein said denitrogenizing flux includes calcium oxide (CaO) and at least one compound selected from the group consisting of oxides, silicates, carbonates of alkali and alkaline earth metals and oxides, fluorides, silicates and carbonates of metals selected from the group consisting of Calcium (Ca), Silicon (Si), Magnesium (Mg), Boron (B), Titanium (Ti), Barium (Ba) and Aluminum (Al).
 8. A method of denitrogenizing an amount of molten metal, comprising steps of: (a) providing an amount of molten metal; and (b) introducing a denitrogenizing flux into the molten metal in such a way that the flux becomes exposed to the molten metal at a location that is at a depth that is substantially below the top surface of the molten metal, thereby promoting more efficient mixing of the flux into the molten metal.
 9. A method according to claim 8 , wherein said denitrogenizing flux includes calcium oxide (CaO) and at least one compound selected from the group consisting of oxides, silicates, carbonates of alkali and alkaline earth metals and oxides, fluorides, silicates and carbonates of metals selected from the group consisting of Calcium (Ca), Silicon (Si), Magnesium (Mg), Boron (B), Titanium (Ti), Barium (Ba) and Aluminum (Al).
 10. A method of introducing a denitrogenizing flux to an amount of molten metal, comprising steps of: (a) supplying an amount of denitrogenizing flux into a lance assembly of the type that includes a nozzle that is constructed and arranged to be immersed in molten metal; and (b) using the lance assembly to introduce the flux into the molten metal.
 11. A method according to claim 10 , wherein said denitrogenizing flux includes calcium oxide (CaO) and at least one compound selected from the group consisting of oxides, silicates, carbonates of alkali and alkaline earth metals and oxides, fluorides, silicates and carbonates of metals selected from the group consisting of Calcium (Ca), Silicon (Si), Magnesium (Mg), Boron (B), Titanium (Ti), Barium (Ba) and Aluminum (Al). 